Preparation of suspensions

ABSTRACT

A method for preparing a suspension of LDH particles comprising the steps of: preparing LDH precipitates by coprecipitation to form a mixture of LDH precipitates and solution; separating the LDH precipitates from the solution; washing the LDH precipitates to remove residual ions; mixing the LDH precipitates with water; and subjecting the mixture of LDH particles and water to a hydrothermal treatment step by heating to a temperature of from greater than 80° C. to 150° C. for a period of about 1 hour to about 144 hours to form a well dispersed suspension of LDH particles in water, wherein said LDH particles in suspension comprise platelets having a maximum particle dimension of up to 400 nm.

FIELD OF THE INVENTION

The present invention relates to a method for preparing a suspension.The present invention also extends to a suspension. In some embodiments,the present invention relates to a stable suspension containing layereddouble hydroxide particles and to a method for preparing suchsuspensions.

BACKGROUND TO THE INVENTION

Layered double hydroxides (hereinafter referred to as “LDHs”) are mixedhydroxides of divalent and tri-valent metals having an excess ofpositive charge that is balanced by interlayer anions. They can berepresented by the general formula (1):

M^(II) _(1-x)M^(III) _(x)(OH)₂A^(n−) _(x/n) .yH₂O  (1)

where M^(II) and M^(III) are di- and tri-valent metal ions respectivelyand A^(n−) is the interlayer anion of valance n. The x value representsthe proportion of trivalent metal to the total amount of metal ionpresent and y denotes variable amounts of interlayer water.

Common forms of LDH comprise Mg²⁺ and Al³⁺ (known as hydrotalcites) andMg²⁺ and Fe³⁺ (known as pyroaurites), but LDHs containing other cationsincluding Ni, Zn, Mn, Ca, Cr, and La are known. The amount of surfacepositive charge generated is dependent upon the mole ratio of the metalions in the lattice structure, and the conditions of preparation as theyaffect crystal formation.

LDH compounds are of interest because they are considered to be usefulas catalysts, catalyst precursors, catalyst supports, absorbents, anionexchangers, PVC stabilisers, flame retardants, medicinal antacids and asmaterial for use in nanocomposites.

LDH particles are typically plate like in morphology. During preparationof LDH compounds, the plate like particles tend to aggregate together toform larger particles, typically having particle sizes in the range ofmicrons or above. It has been found to be difficult to disperse theaggregated particles because of the strong interactions between theplaty LDH nanosheets, such as electrostatic attraction via the commonsurface anions and hydrogen-bonds via water molecules.

LDHs can be prepared by forming a mixed solution containing the M²⁺ andM³⁺ ions in solution and adjusting the pH of the solution to an alkalinepH. This results in the coprecipitation of the LDH as solid particles.Other synthetic pathways to form LDHs, particularly those containingmagnesium, include synthesis from Mg(OH)₂ (brucite) and MgO (calcinedmagnesia) via incorporating trivalent metal ions, such as Al³⁺, andincluding anions. A number of other methods for producing LDHs have alsobeen described.

Liu et al, “Liquid—Crystalline Phases of Colloidal Dispersions ofLayered Double Hydroxides”, Chem. Mater. 2003, 15, 3240-3241, describedthe synthesis of colloidal Mg/Al LDH that was carried out using anon-steady coprecipitation method. The pH of an aqueous solution ofmixed magnesium and aluminium chlorides was raised to 9.5 by adding 3.5M NH₃.H₂O under vigorous stiffing. The resulting precipitate was aged atroom temperature for one hour. After filtration, the filter cake waswashed thoroughly with deionised water. It was then collected and closedin a glass bottle for peptization in a thermostat at 80° C. for 24hours. Well dispersed colloidal LDH particles were obtained.Transmission electron microscopy showed that most particles were roughlymonodispersed platelets, hexagonal in shape, with diameters between 50and 80 nm, and the electron diffraction pattern showed Mg/Al LDHparticles were well-crystallined with hexagonal symmetry. The particlethickness of about 5 nm was also revealed. The Zeta potential of theparticles was measured to be +39 millivolts. Dispersions of the LDHs inwater were shown to form liquid crystalline phases.

J.-M. Oh et al, “The Effect of Synthetic Conditions on Tailoring theSize of Hydrotalcite Particles”, Solid State Ionics, 151 (2002),285-291, investigated preparation of hydrotalcites. In particular, clearmetal solutions containing magnesium and aluminium were titrated up topH of approximately 11 with sodium hydroxide solution containing sodiumcarbonate and aged in an autoclave at 100° C. for 12, 24, 48, 72 hours,and also at 100° C., 125° C., 150° C., 180° C., for 48 hoursrespectively. The particle sizes were analysed and it was found thatincreasing aging time and increasing temperature result in increasingparticle size. The average particle size ranged from 85 nm (for aging at100° C. for 12 hours) to 340 nm (for aging at 180° C. for 48 hours).

European patent application no. 987328 in the name of Jin Ho Choydescribed a bio-inorganic hybrid composite for retaining and carryingbio-materials with stability and reversible dissociativity. Thebio-inorganic hybrid composite was prepared by forming a stable layereddouble hydroxide in which anions are intercalated and subjecting theintercalated anions to an ion exchange reaction with a bio material. Thebio material is suitably nucleoside-5′ monophosphate,nucleoside-5′triphosphate, or a gene material with a size of 500-1000base pairs.

The applicant does not concede that the prior art discussed above formspart of the common general knowledge in Australia or elsewhere.

Throughout this specification, the word “comprising” or its grammaticalequivalents shall be taken to have an inclusive meaning unless thecontext clearly indicates otherwise.

BRIEF DESCRIPTION OF THE INVENTION

In a first aspect, the present invention provides a method for preparinga suspension of LDH particles comprising the steps of:

a) preparing LDH precipitates by coprecipitation to form a mixture ofLDH precipitates and solution;

b) separating the LDH precipitates from the solution;

c) washing the LDH precipitates to remove residual ions;

d) mixing the LDH precipitates with water; and

e) subjecting the mixture of LDH precipitates and water from step (d) toa hydrothermal treatment step by heating to a temperature of greaterthan 80° C. to 150° C. for a period of about 1 hour to about 48 hours toform a suspension of LDH particles in water, wherein the LDH particleshave a maximum particle dimension of up to 400 nm and exhibit a particlesize distribution of no more than about ±27%, based upon the peak valueand the peak width at half maximum from the particle size distributionof an intensity average using photon correlation spectroscopy (PCS)measurement.

In a second aspect, the present invention provides a method forpreparing a suspension of LDH particles comprising the steps of:

a) preparing LDH precipitates by coprecipitation to form a mixture ofLDH precipitates and solution;

b) separating the LDH precipitates from the solution;

c) washing the LDH precipitates to remove residual ions;

d) mixing the LDH precipitates with water; and

e) subjecting the mixture of LDH particles and water from step (d) to ahydrothermal treatment step by heating to a temperature of from greaterthan 80° C. to 150° C. for a period of about 1 hour to about 2 hours toform a well dispersed suspension of LDH particles in water, wherein theLDH particles have a maximum particle dimension of up to 400 nm.

Suitably, the hydrothermal treating step is carried out whilstsuppressing boiling.

The LDH may have a composition as given in formula (1) above. Thisformula may also be written as formula (2):

M^(II) _(n)M^(III)(OH)_(2(n+1))X.yH₂O  (2)

wherein X=one or more anions or negatively charged material to balancecharge in the hydroxide layer. X is typically present in the interlayerspace in the LDH material.

M^(II) is suitably Mg, although other metal ions of valence 2+ may alsobe used. M^(III) is suitably Al. It will be appreciated that other metalions of valence 3+ may also be used. Examples of other metal ions thatmay be used include:

M^(II): Fe, Co, Ni, Cu, Zn, Mn, Pd, Ti, Cd and Ca

M^(III): Co, Fe, Mn, Ga, Rh, Ru, Cr, V, In, Y, Gd and La.

These lists should not be considered to be limiting.

The coprecipitation step suitably involves the steps of forming a mixedmetal ion solution containing the appropriate metal ions and adding thatsolution into an alkaline material to form LDH precipitates. Suitably,the alkaline material is an alkaline solution. Precipitation of layereddouble hydroxides typically occurs when the pH of the mixed metal ionsolution is raised to greater than 6-7 depending on the metal ions used.The alkaline solution that is used in the present invention is suitablya sodium hydroxide solution, together with either sodium bicarbonate orsodium carbonate if necessary. However, it will be appreciated thatnumerous other alkaline solutions, such as ammonia solutions, KOH,NaHCO₃, KHCO₃, Na₂CO₃, K₂CO₃, and possibly some organic amines, such asmethylamine, ethylamine may also be used in the process of the presentinvention. This list should not be considered to be exhaustive and otheralkaline solutions may also be used in the present invention.

As will be well known to the person skilled in the art, the solutionsare suitably stirred or agitated during the mixing and precipitationsteps.

The mixed metal ion solution may suitably be prepared by dissolvingappropriate salts of the metals in water. The metal salts are, forexample, chlorides, nitrates, sulfates or any other metal salts that arereadily soluble and inexpensive. Alternatively, the appropriate metalsmay be placed in acid solutions to be thereby dissolved to form themixed metal ion solution. Acids that may be used include hydrochloricacid (HCl), nitric acid (HNO₃), sulfuric acid (H₂SO₄) as well as manyother organic carboxylic acids, such as methanoic acid, acetic acid.

The coprecipitation step suitably involves adding the mixed metal ionsolution to an alkaline solution with appropriate stirring or agitation.It is preferred that the mixed metal ion solution and the alkalinesolution are rapidly mixed together. Suitably, the mixed metal ionsolution and the alkaline solution are mixed together within a timeperiod of less than 1 minute, more suitably within a time period of lessthan 20 seconds, even more suitably in a time period of less than 5seconds, most suitably in a time period of less than 2 seconds.

Precipitation of the LDH is rapid once the pH has risen to the requiredlevel. Therefore, rapid mixing of the mixed metal ion solution and thealkaline solution results in precipitation of the LDH precipitates overa short period of time.

The precipitation step may take place at room temperature, althoughother temperatures may be used during the coprecipitation step. Indeed,temperatures of up to 50° C. during the precipitation step have beenshown to have little effect on the particle size distribution obtainedby the present invention.

Once the LDH precipitates have precipitated, a mixture of the LDHprecipitates and solution is obtained. This mixture has a pH thatdepends upon the ions used and the relative amount of alkaline materialadded. For MgAl-LDH, the pH is generally 7.5-8.0. The particles and thesolution are suitably left in contact with each other for a period of nomore than 30 minutes following precipitation of the LDH precipitates.More suitably, the LDH precipitates and the solution are left in contactwith each other for a period not exceeding 20 minutes, even moresuitably not exceeding 10 minutes, yet more suitably not exceeding 5minutes and most suitably left in contact for a period not exceeding 1minute. During such contact, vigorous stirring is generally applied.

The LDH precipitates and the solution are then separated from eachother. Any suitable method of separating solid particles from a liquidsolution may be used. Examples include centrifugation and filtration.

Following separation, the LDH precipitates are washed. The washing stepremoves any residual ions in the mixed solution. It also removes anyresidual alkaline material. The LDH precipitates may be washed one or,more preferably, two or more times to ensure that substantially allresidual ions are removed from the LDH precipitates. De-ionized water issuitably used to wash the LDH precipitates.

The LDH precipitates are then mixed with water. Suitably, theprecipitates are dispersed in water, especially de-ionized water.Suitably, the LDH precipitates are dispersed in water by the vigourousmixing of the precipitates in water. Ideally, this mixing step willbreak up any loose aggregates of LDH precipitates that may have beenformed in the precipitation step. This mixture is then subjected to thehydrothermal treating step (step (e)). The hydrothermal treating step isconducted by heating the mixture of LDH precipitates and water to anelevated temperature whilst, in most cases, suppressing boiling. Thetemperature in the hydrothermal treating step is from greater than 80°C. to 150° C. Suitably, the temperature used in the hydrothermaltreating process is between 80° C. and 150° C., more suitably between 80and 110° C. and most suitably at about 100° C. The mixture of water andLDH particles is suitably held at the elevated temperature for a periodof from 1 to 48 hours, more suitably from 1-24 hours. A well dispersedsuspension of LDH particles in water is obtained following thehydrothermal treatment step.

Suppression or prevention of boiling, if required, is typically achievedby maintaining the pressure in the hydrothermal treating stepssufficiently high to stop boiling. Suitably, the pressure in thehydrothermal treating step is the autogenous pressure of the mixture atthe elevated temperature used, usually less than 5 atm. It will beappreciated that the hydrothermal treating step may be conducted in apressure vessel.

The inventors have surprisingly found that preparing a LDH suspension inaccordance with some embodiments of the first aspect of the presentinvention results in the formation of a stable suspension containing LDHparticles in the form of dispersed platelets. The largest dimension ofthe particles in such stable suspensions predominantly falls within therange of 20-400 nm, more suitably 40-300 nm, with the thickness of theparticles predominantly falling within the range of 5-40 nm. Theparticles also exhibit a narrow particle size distribution, with theparticles typically showing a particle size distribution of ±27% aroundthe average size. A reduced tendency towards aggregation has beennoticed by the present inventors.

The LDH particles typically have an aspect ratio that falls within therange of from 5 to 10. In this regard, “aspect ratio” relates to theratio of the largest dimension of the particle to its thickness orheight.

The inventors have found that suspensions that include particles with alargest particle dimension of up to 400 nm, more suitably from 20 to 300nm, form stable suspensions that do not exhibit separation orsegregation.

LDH suspensions made in accordance with the method of the presentinvention may have up to 10% w/w LDH particles, suitably, up to 5% w/wLDH particles, even more suitably about 1% w/w LDH particles, mostsuitably less than 1% w/w LDH particles.

In a third aspect, the present invention provides a suspension of LDHparticles in water comprising LDH particles in the form of plateletshaving a maximum particle dimension falling predominantly in the rangeof 20-400 nm, a particle concentration of up to 10% w/w, with thesuspension exhibiting no settling or segregation for a period of atleast one month from formation.

The particles suitably exhibit a narrow particle size distribution, withthe particles typically showing a particle size distribution of ±27%around the average size of the particles. For example, if the averagesize of the particles is 100 nm, the particles would mostly range from73 nm to 127 nm.

The suspension may suitably have a particle concentration of up to 10%w/w, suitably 5% w/w, more suitably about 1% w/w, even more suitablyless than 1% w/w. The maximum particle dimension of the particlessuitably falls in the range of 20 to 400 nm. It has been found that somesuspensions containing around 10% w/w particles may exhibit a smalldegree of settling, possibly due to aggregation caused by “crowding” ofthe particles or due to insufficient dispersion of the particles.

The method of the first aspect of the present invention may furtherinclude the step of removing at least some of the water from thesuspension to concentrate the particles. This may form a moreconcentrated suspension (i.e. one having a higher loading of particles)or even lead to the recovery of LDH particles from the suspension. Thewater may be removed by any suitable step known to the skilled person,such as by drying, filtration or by centrifugation followed by removalof the supernatant liquid layer. The separated LDH particles may besubsequently dried.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic diagram of a typical individual particle of alayered double hydroxide;

FIG. 2 shows a schematic diagram demonstrating aggregation of LDHparticles;

FIG. 3 shows TEM images of LDH particles from a suspension in accordancewith an embodiment of the present invention and from an ethanolsuspension of LDH prepared by a conventional method;

FIG. 4 shows the particle size distribution of Mg₂Al—Cl-LDH samples: (X)stirred for 10 minutes at room temperature, and then dispersed withultrasonic treatment for 20 minutes, with two peaks at 320 nm and 2300nm; (Y) aged at 50° C. for 18 hours, and dispersed with ultrasonictreatment for 20 minutes, with two broad peaks at 220 nm and 955 nm; (Z)as-prepared with the current method, with one sharp peak at 114 nm;

FIG. 5 shows the dispersion of Mg₂Al—Cl-LDH aggregates with heatingduration during the hydrothermal treatment at 100° C.;

FIG. 6 shows the relationship between primary particle size of the LDHparticles and duration of the hydrothermal treatment step, forhydrothermal treatment steps utilizing a temperature of 100° C.;

FIG. 7 shows the particle size distribution of LDH samples producedusing a hydrothermal heating step having a temperature of 100° C. andheating time of from 1 hour to 2 hours;

FIG. 8 shows the particle size distribution of LDH samples producedusing a hydrothermal heating step having a temperature of 150° C. andheating time of from 1 hour to 2 hours; and

FIG. 9 shows the particle size distribution of LDH samples producedusing a hydrothermal heating step having a temperature of 80° C. andheating time of from 8 hour to 16 hours.

DETAILED DESCRIPTION OF THE DRAWINGS

It will be appreciated that the drawings attached to this specificationhave been provided for the purposes of illustrating preferredembodiments of the present invention.

FIG. 1 shows a schematic diagram of a typical LDH particle. The typicalLDH structure has metal hydroxide sheets that carry a net positivecharge due to limited substitution of trivalent cations for divalentcations. As the structure includes metal hydroxide sheets, separatesheets tend to form layers one upon another, with anions or othernegatively charged material being positioned between the sheets in orderto balance the charge. Where anions are present between the sheets, theyare often referred to interlayer anions. It has been found thatinterlayer anions may be exchanged or substituted.

LDH particles exhibit preferable growth along the a and b axis (as shownin FIG. 1) to form hexagonal platy sheets having an aspect ratiotypically of 5-10. As can be seen from FIG. 1, the LDH particle shownschematically in FIG. 1 comprises layers 12, 14, 16, 18, 20 of hydroxidesheets. Each layer represents one positively charged brucite-likehydroxide layer (formula: M^(II) _(n)M^(III)(OH)_(2(n+1)) ⁺) and betweentwo layers are anions and water molecules (X⁻.yH₂O)_(z), Although FIG. 1shows five layers, it will be appreciated that the LDH particles maycontain a greater or lesser number of brucite-like hydroxide layers. Ingeneral, there may be 5-150 such brucite-like hydroxide layers incrystalline LDH particles. Such LDH particles typically have a particlesize ranging from about 30-1000 nm in the largest particle dimension,with a thickness typically from about 5-100 nm.

FIG. 2 shows a schematic diagram of aggregation of individual LDHnanoparticles. It has previously been found that LDH particles tend toaggregate together when placed in suspension to form larger particles,typically having a particle size in the range of 1-10 microns. Forexample, the aggregated particle 22 shown schematically in FIG. 2comprises an aggregation of a plurality of individual particles, some ofwhich are numbered as 24, 26, 28. (Each of these individualnanoparticles is typically composed of a number of brucite-likehydroxide layers, as shown in FIG. 1.) It has been postulated thataggregation of individual LDH particles to form an aggregated particleoccurs due to the following:

1) the sheets of hydroxides on the top and bottom surfaces of theindividual particles are typically positively charged and also typicallyhave anions or other negatively charged material associated therewith tobalance the charge. It is possible that the surface anions or surfacenegatively charged material may be shared between the overlapping partof adjacent particles;

2) the individual LDH particles have some defects so that they mightshare some top points and edges of lattice cells; and

3) amorphus or very small particles may act as a glue between theindividual platy particles.

It has been found that the aggregation of the particles causes settlingof the LDH particles from suspension.

The inventors have found that the method in accordance with the firstaspect of the present invention produces a suspension in which the LDHparticles are dispersed and exhibit little tendency to aggregate.Accordingly, suspensions in accordance with the present invention tendto exhibit little or no segregation or settling for extended periods oftime, for example, for at least one month and, in some instances, up to6 months. Without wishing to be bound by theory, the inventors havepostulated that the hydrothermal treatment step of the method of thepresent invention has the following effects:

a) The higher temperatures present during the hydrothermal treating stepmay provide extra kinetic energy for the individual LDH nanoparticles toundergo stronger Brownian motion to collide with aggregates and breakaggregates into individual nanoparticles. Once these nanoparticles areseparated individually, they are kept apart from one another due to theelectrostatic repulsion between these nanoparticles, because thesenanoparticles overall carry positive charges (zeta potential is 30-50mV);

b) in the hydrothermal treatment step, the LDH particles become moreperfect because the higher temperatures in the hydrothermal treatingstep allow the M^(II) ions and M^(III) ions in the hydroxide sheets tomove around to a more desired distribution. Thus, the positive charge ineach hydroxide sheet becomes more evenly distributed, thus reducing thesharing of surface anions by adjacent LDH particles;

c) some of the smaller amorphus particles probably dissolve during thehydrothermal treatment step to promote the growth of LDH crystallites.

Each of the above postulated mechanisms would act to reduce aggregationof the individual LDH particles.

The inventors have also found that there may also be benefits obtainedby causing the precipitation of the LDH particles to occur very quickly.When precipitation processes take place, two processes act to form theparticles, namely nucleation and growth. In nucleation, very smallparticles nucleate from the solution once appropriate precipitatingconditions have been reached. Nucleation of particles typically takesplace on surface defects in the vessel in which precipitation isoccurring or on impurities in the solution, such as dust particles inthe solution. Seed particles may also be used. Further precipitation canthen occur by virtue of growth, in which precipitating solids aredeposited on the nucleated particles to increase the particle size ofthe particles. In preferred embodiments of the present invention,precipitation occurs by virtue of very rapid mixing of the mixed metalion solution with an alkaline solution. For example, the mixed metal ionsolution and an alkaline solution may be fully mixed together in lessthan a minute, more preferably less than 20 seconds, even morepreferably less than 5 seconds, most preferably with the mixed metal ionsolution and alkaline solution being mixed together in less than 2seconds. This rapid mixing causes a high rate of nucleation of smallparticles.

It will also be appreciated that vigorous stirring during mixing of themixed metal ions solution and the alkaline solution is likely to assistin preparing LDH particles having a narrow particle size distribution.It is believed that vigorous stirring may help to evenly spread themetal ions in the alkaline solution, promote the precipitation of LDHparticle to occur homogeneously, and lead to a uniform LDH particle sizedistribution. The uniformity in LDH particles at this stage isadvantageous to the stability of the suspension and uniformity of LDHparticle size after the hydrothermal treatment.

It is further preferred that the mixture of precipitated particles andsolution obtained from mixing the mixed metal ion solution and alkalinesolution remains together for not more than 30 minutes after initialmixing. Without wishing to be bound by theory, the inventors havepostulated that an aging phenomenon occurs when the precipitatedparticles and the solution (which is still at a slightly alkaline pH forMgAl-LDH, for example) remain together. This aging phenomenon causesredistribution of lattice ions in the hydroxide, growth of the particlesand aggregation of the particles. The present inventors have furtherpostulated that minimising the length of time of contact between thesolution and the precipitated LDH particles minimises this agingphenomenon and thereby minimises growth and aggregation of the LDHparticles. It is believed that this further enhances the beneficialeffects of the hydrothermal treatment step.

The inventors have postulated that the rapid mixing of the salt solutionand the alkaline solution within a very short time period provides anequal opportunity for each metal ion to precipitate at the same time andminimises the time for the nucleates to grow, thus resulting inrelatively uniform primary LDH crystallites, which assist in obtainingmonodispersed LDH particles after the hydrothermal treatment.

Example 1

The following procedure was used to prepare a suspension in accordancewith the present invention:

-   -   1) Prepare 10 mL salt solution containing 0.3 M MgCl₂, 0.1 M        AlCl₃. (Solution A);    -   2) Prepare 40 mL 0.15 M NaOH solution (Solution B);    -   3) Add solution A into solution B within 2 seconds to        precipitate under vigorous stirring;    -   4) Stir at room temperature for 10 min;    -   5) Separate via centrifugation;    -   6) Wash two times with deionized water via centrifugation;    -   7) Manually disperse the LDH slurry in 40 mL water and place        into an autoclave;    -   8) Hydrothermally treat the suspension in the autoclave at        100° C. for 8 hrs;    -   9) Cool down the autoclave to room temperature and store the        suspension.        This suspension contained about 0.4% w/w Mg₂Al(OH)₆Cl.H₂O, with        the product yield being about 60%.

FIG. 3 shows a photomicrograph of a suspension made in accordance withthe above procedure, which shows well dispersed, hexagonal shapedparticles. FIG. 3 also shows a photomicrograph of an LDH suspensionprepared by a conventional method and then dispersed in ethanol. Thisshows much less well dispersed particles that still exhibit apparentaggregation.

Example 2

The following procedure was used to prepare a suspension in accordancewith the present invention:

-   -   1) Prepare 10 mL salt solution containing 0.2 M Co(NO₃)₂, 0.1 M        Al(NO₃)₃. (Solution A);    -   2) Prepare 40 mL alkaline solution 0.15 M NaOH and 0.013 M        Na₂CO₃ (Solution B);    -   3) Add solution A into solution B within 2 seconds to        precipitate under vigorous stirring;    -   4) Stir at room temperature for 30 min;    -   5) Separate via centrifugation (pH≈10);    -   6) Wash two times with deionzed water via centrifugation;    -   7) Manually disperse the LDH slurry in 40 mL water and place        into an autoclave;    -   8) Hydrothermally treat the suspension in the autoclave at        100° C. for 8 hrs;    -   9) Cool down the autoclave to room temperature and store the        suspension.        This suspension contains about 0.5% w/w        Co₂Al(OH)₆(CO₃)_(0.5).H₂O, the product yield is about 60%.

Example 3

The following procedure was used to prepare a suspension in accordancewith the present invention:

-   -   1) Prepare 10 mL salt solution containing 0.3 M Mg(NO₃)₂ and 0.1        M Al(NO₃)₃ (Solution A);    -   2) Prepare 40 mL 0.15 M NaOH (Solution B);    -   3) Add 10 mL solution A into 40 mL solution B to precipitate        under vigorous stirring;    -   4) Stir at room temperature for 10 min;    -   5) Separate via centrifugation;    -   6) Wash two times with deionized water via centrifugation;    -   7) Disperse the slurry in 40 mL water and place into an        autoclave (45 mL);    -   8) Heat the autoclave at 100° C. for 16 hrs.    -   9) Cool down the autoclave to room temperature and store the        suspension.        This suspension contains about 0.4% w/w Mg₂Al(OH)₆NO₃.H₂O, the        product yield is about 60%.

Example 4

The following procedure was used to prepare a suspension in accordancewith the present invention:

-   -   1) Prepare 10 mL salt solution containing 0.3 M MgCl₂ and 0.1 M        AlCl₃ (Solution A);    -   2) Prepare 40 mL 0.15 M NaOH and 0.013 M Na₂CO₃ (Solution B);    -   3) Add 10 mL solution A into 40 mL solution B to precipitate        under vigorous stirring;    -   4) Stir at room temperature for 10 min;    -   5) Separate via centrifugation;    -   6) Wash two times with deionized water via centrifugation;    -   7) Disperse the slurry in 40 mL water and place into an        autoclave (45 mL);    -   8) Heat the autoclave at 100° C. for 16 hrs.    -   9) Cool down the autoclave to room temperature and store the        suspension.        This suspension contains about 0.45% w/w        Mg₃Al(OH)₈(CO₃)_(0.5).H₂O, the product yield is about 60%.

Example 5

The following procedure was used to prepare a suspension in accordancewith the present invention:

-   -   1) Prepare 10 mL salt solution containing 0.3 M MgCl₂, 0.04 M        FeCl₃ and 0.06 M AlCl₃ (Solution A);    -   2) Prepare 40 mL 0.15 M NaOH (Solution B);    -   3) Add 10 mL solution A into 40 mL solution B to precipitate        under vigorous stirring;    -   4) Stir at room temperature for 10 min;    -   5) Separate via centrifugation;    -   6) Wash two times with deionized water via centrifugation;    -   7) Disperse the slurry in 40 mL water and place into an        autoclave (45 mL);    -   8) Heat the autoclave at 100° C. for 16 hrs.    -   9) Cool down the autoclave to room temperature and store the        suspension.        This suspension contains about 0.4% w/w        Mg₂Fe_(0.04)Al_(0.06)(OH)₂Cl.H₂O, the product yield is about        60%.

Example 6

The following procedure was used to prepare a suspension in accordancewith the present invention:

-   -   1) Prepare 10 mL salt solution containing 0.3 M MgCl₂ and 0.1 M        AlCl₃ (Solution A);    -   2) Prepare 40 mL 0.15 M NaOH (Solution B);    -   3) Add 10 mL solution A into 40 mL solution B to precipitate        under vigorous stirring;    -   4) Stir at room temperature for 10 min;    -   5) Separate via centrifugation;    -   6) LDH slurry is exchanged with Na₂SO₄ (40 mL 0.05 M) for 30        min;    -   7) Separation and then wash 1 time;    -   8) Disperse the slurry in 40 mL water and place into an        autoclave (45 mL);    -   9) Heat the autoclave at 100° C. for 16 hrs.    -   10) Cool down the autoclave to room temperature and store the        suspension.        This suspension contains about 0.4% w/w        Mg₂Al(OH)₆(SO₄)_(0.5)H₂O, the product yield is about 60%.

FIG. 4 shows a graphical representation of the particle sizedistribution of suspensions of LDH particles prepared in accordance withthe present invention (line Z in FIG. 4), the particle size distributionof freshly prepared LDH precipitates that have been subjected toultrasonication for 20 minutes (i.e. no hydrothermal treatment, Line Xin FIG. 4) and the particle size distribution of fresh LDH precipitatesthat have been subjected to ultrasonication for 20 minutes and agedovernight at 50° C. (line Y in FIG. 4). The particle size distributionwas measured using photon correlation spectroscopy (PCS).

The Mg₂Al—Cl-LDH suspension obtained after hydrothermal treatment at100° C. for 16 hours has a narrow particle size distribution with anequivalent hydrodynamic diameter of 114 nm, with all particlesinclusively within 45-250 nm. However, the suspensions of the same LDHmaterial made conventionally without a hydrothermal treatment anddispersed with the assistance of ultrasonication in water have a muchwider particle size distribution and larger diameters (200-3000 nm). Inparticular, the suspension of freshly precipitated Mg₂Al—Cl-LDH, afterultrasonication for 20 min, consists of a bimodal particle sizedistribution, with diameters at 320 nm and 2300 nm, respectively. Afteraging at 50° C. overnight, the aggregates decrease in size to 220-955nm. This means that the aggregates can be only partially segregatedafter aging and/or ultrasonication. However, the inventor's experimentshave demonstrated that these partially segregated particles can be alsosegregated further into individual nanoparticles by the process of thepresent invention. Visually, the well-dispersed suspension looks verytransparent while the conventional ones are turbid. These evidencessuggest that the aggregates are completely segregated and well dispersedinto much smaller particles after the hydrothermal treatment at 100° C.for 16 hours.

FIG. 5 shows a graphical representation of particle size distributionfor LDH suspensions subjected to hydrothermal treatments of differingduration and a hydrothermal treatment temperature of 100° C. As can beseen from FIG. 5, after hydrothermal treatment at 100° C. for a suitableperiod of time (4, 8, 16 or 48 hours), the LDH particles in as-madeMg₂Al—Cl-LDH suspensions become very uniform, with a narrow particlesize distribution. However, the 2-hour or 144-hour treatment leads toextra distribution bands with much bigger particles (FIG. 5), indicatingthe presence of some degree of aggregation. It seems that 2-hour heattreatment at 100° C. does not disperse all aggregates into individualLDH crystallites. Compared with the freshly prepared LDH suspension(FIG. 4, curve X), 2-hour treated LDH suspension contains much smallerparticles (69 vs. 320 nm), which further suggests that the freshlyprepared LDH aggregates are only partially segregated withultrasonication. However, there are two groups of bigger particles withsize of 800 nm and over 4 μm, respectively. Subsequently, these twogroups of particles disappeared when the treatment time is extended to 4hours, resulting in one narrow particle size distribution band (89 nm)in the LDH suspension. This suggests that 2-hour treatment at 100° C. isnot sufficient to fully disperse the aggregates while the 4-hourtreatment completes the segregation of LDH particles. As the treatmenttime increases further, the LDH particle size distribution band isshifted to the big particle size side, indicating the continuous growthof LDH particles with time. However, if the hydrothermal treatment iscontinued to 144 hours at 100° C., the LDH particle size distributionband becomes quite broad and the LDH crystallite size is hundreds ofnanometers. As a consequence, a weak and broad band is seen around 5 μm,presumably due to the re-aggregation of these bigger LDH crystallites.

The above results suggest that, for the shorter hydrothermal treatmenttimes in the present invention, temperatures in the upper part of thetreatment temperature range should be used, whilst at the longerhydrothermal treatment times, temperatures in the lower part of therange should be used.

Regardless of the aggregates, the primary Mg₂Al—Cl-LDH particle size,i.e. the peak value from PCS curves in FIG. 5 constantly increase from70 to 300 nm with increasing the hydrothermal treatment time from 2 to144 hours, as clearly shown in FIG. 6 (bold curve). The relationship isalmost linear, indicating a rough growth rate of 1.5 nm per hour. It istherefore concluded that the Mg₂Al—Cl-LDH particle size can be tailoredin the range of 70-300 nm by adjusting hydrothermal treatment time at100° C.

In the case of Mg₂Al—CO₃-LDHs (i.e. CO₃ being the interlayer anion), therelationship between the treatment time and the LDH particle size isquite similar. As shown in FIG. 6 (thin curve), the hydrodynamicdiameter of LDH-OO₃ particles almost linearly increase from 45 to 118 nmwith the time from 4 to 72 hours in the logarithmic scale. Veryinterestingly, the average particle size of LDH-OO₃ is smaller roughlyby 40-50 nm than that of LDH-Cl at every identical time point,suggesting a similar growth pattern but different growth preference inthese two systems.

The effect of hydrothermal treatment temperature was also investigated.Table 1 shows the particle size of Mg₂Al—Cl-LDH under the varioushydrothermal treatments as specified in Table 1.

TABLE 1 Mg₂Al-LDH-Cl particle size (nm) under different conditions. time80° C. 100° C. 125° C. 150° C. 2 hr   69 ± 15  89 ± 19  94 ± 20  (465 ±110) 4 hr   99 ± 36 ^(a)  89 ± 16 111 ± 22 118 ± 18  (2500 ± 660) ^(b)(3300 ± 540) 8 hr 87 ± 19 101 ± 19 124 ± 30 160 ± 30 (4600 ± 420) 16 hr 92 ± 18 114 ± 19 162 ± 46 194 ± 34 (4200 ± 500) (4300 ± 520) 48 hr  159± 29 144 hr  284 ± 91 (4200 ± 500) Note: ^(a) The peak value and thepeak width at half maximum of small particles are from the particle sizedistribution on intensity average. ^(b) The data in the parenthesis arecorresponding to big size particles that are not dispersed orre-aggregated.

The treatment temperature seems to more strongly influence LDH particlesize than treatment time. As given in Table 1 for Mg₂Al—Cl-LDHs, anincrease in temperature by 10° C. can lead to an increase in thehydrodynamic diameter by 10-15 nm on average after hydrothermaltreatment for 8 or 16 hours, or the primary particle size is doubledwhen temperature increases from 80 to 150° C. for treatment time of 8 or16 hours. This shows the quick growth of LDH crystallites at highertemperatures. In contrast, if treatment time is short, such as 2 or 4hours, the particle growth seems to be much slower (3-5 nm per 10° C.).This comparison suggests that in the beginning of hydrothermaltreatment, the major event is segregation of LDH aggregates toindividual LDH crystallites in the aqueous suspension. Subsequently LDHcrystallites grow continuously with time.

Similarly, incomplete dispersion is observed at lower temperatures forshort treatment duration while re-aggregation takes place at highertemperatures even at short treatment time (see Table 1). For example,the treatment at 80° C. for 4 hours does not disperse all the aggregateswhile the treatment at 150° C. for 4 hours is long enough to producebigger LDH crystallites to re-form the micrometer-scaled aggregates.

A number of LDH suspensions were prepared in accordance with the presentinvention. Table 2 summarises the conditions under which the suspensionswere prepared and gives the average particle size and the measured zetapotentials.

TABLE 2 Summary of Mg₂Al-LDH Particle size and Zeta potential Particlesize ξ Potential Compounds ^(a) Conditions (nm) (mV) Mg₂Al—Cl-LDH 100°C., 4 h, 0.4%^(b) 89, 95, 97, 51.9 87, 92^(c) 100° C., 8 h, 0.4% 10148.3 100° C., 16 h, 0.4% 114 47.0 100° C., 48 h, 0.4% 159, 159, 49.1171, 160 100° C., 144 h, 0.4% 284 47.9 100° C., 16 h, 1.0% 101 53.0 100°C., 16 h, 2.0% 116 51.3 100° C., 16 h, 3.0% 115 45.0 100° C., 16 h, 4.0%120 50.4 100° C., 72 h, 1.0% 155 — 80° C., 8 h, 1.0%  85 38.5 80° C., 16h, 1.0%  89 36.9 Mg₂Al—CO₃-LDH 100° C., 4 h, 0.4% 46, 43 39.5, 48.5 100°C., 8 h, 0.4%  58 37.2 100° C., 16 h, 0.4% 75, 71, 68 42.0, 48.9 100°C., 72 h, 0.4% 118 48.5 Mg₂Al—NO₃-LDH 100° C., 16 h, 0.4% 112, 117, —119 Mg₂Al—SO₄-LDH 100° C., 16 h, 0.4% 142 28.4, 28.1 Mg₃Al—Cl-LDH 100°C., 16 h, 0.4% 106 43.8, 45.7 Mg₃Al—CO₃-LDH 100° C., 16 h, 0.4% 143 33.6Ni₂Al—CO₃-LDH 100° C., 16 h, 0.5% 41, 43 40.6, 41.2 Co₂Al—Cl-LDH 100°C., 16 h, 0.25% 127 37.2, 38.1 Co_(0.5)Mg_(1.5)Al—Cl- 100° C., 16 h,0.2% 125 43.1, 42.8 LDH Mg₂Al_(0.7)Fe^(III) _(0.3)—Cl- 100° C., 16 h,0.4% 123 — LDH Mg₂Al_(0.6)Fe^(III) _(0.4)—Cl- 100° C., 16 h, 0.4% 11037.7, 39.0 LDH Mg₂Al_(0.5)Fe^(III) _(0.5)—Cl- 100° C., 16 h, 0.4%  90 —LDH Notes for Table 2: ^(a) The composition of compound is nominal.^(b)This indicates that the suspension was hydrothermally treated at100° C. for 4 hours, with nominal LDH weight percentage of 0.4% in thesuspension. ^(c)Multiple value was obtained from different repeatedsuspensions

Example 7

A further sample of LDH particles were prepared using the same procedureas given in Example 1, except that the hydrothermal treatment step instep (8) of example 1 was conducted at 100° C. for 1 hour, 1.5 hours and2 hours. Step (7) of example 1, which involved manually dispersing theLDH slurry in 40 mls water and placing the slurry into an autoclave,consisted of hand shaking the water and LDH particles.

The particle size distributions of the suspensions obtained in theseexperimental runs are shown in FIG. 7. The average particle size and theparticle size distribution, determined from the peak value and the peakwidth at half maximum from the particle size distribution of anintensity average using photon correlation spectroscopy (PCS)measurement, are shown in Table 3.

TABLE 3 Sample Preparation Peak size Conditions (nm) 100° C., 1 h 79 ±15 100° C., 1.5 h 77 ± 15 100° C., 2 h 91 ± 17

Example 8

A further batch of experimental runs were conducted using the procedureoutlined in example 7, except that the hydrothermal treatment step instep (8) of example 1 was conducted at 150° C. for 1 hour, 1.5 hours and2 hours. Step (7) of example 1, which involved manually dispersing theLDH slurry in 40 mls water and placing the slurry into an autoclave,consisted of hand shaking the water and LDH particles.

The particle size distributions of the suspension obtained in theseexperimental runs are shown in FIG. 8. The average particle size and theparticle size distribution, determined from the peak value and the peakwidth at half maximum from the particle size distribution of anintensity average using photon correlation spectroscopy (PCS)measurement, are shown in table 4.

TABLE 4 Sample Preparation Peak size Conditions (nm) 150° C., 1 h 121 ±30 150° C., 1.5 h 124 ± 27 150° C., 2 h 139 ± 29

The results obtained in example 8 demonstrate that it is possible toperform the method of the present invention at a temperature of 150° C.whilst still obtaining a narrow particle size distribution (having nosecondary peak in the distribution curve at larger particle sizes). Insome instances where the process of the present invention is operated atthe higher limits of the temperature ranges described herein, it may bedesirable to carefully disperse the precipitates before the hydrothermaltreatment. This assists in ensuring that the final particle sizedistribution does not exhibit secondary peaks at particle sizes above400 nm.

Example 9

The following procedure was used to prepare a suspension in accordancewith the present invention:

-   -   1) Prepare 10 mL salt solution containing 0.6 M MgCl₂, 0.2 M        AlCl₃. (Solution A);    -   2) Prepare 40 mL 0.30 M NaOH solution (Solution B);    -   3) Add solution A into solution B within 2 seconds to        precipitate under vigorous stirring;    -   4) Stir at room temperature for 10 min;    -   5) Separate via centrifugation;    -   6) Wash two times with deionized water via centrifugation;    -   7) Manually disperse the LDH slurry in 30 mL water and place        into an autoclave;    -   8) Hydrothermally treat the suspension in the autoclave at        80° C. for 8 hr;    -   9) Cool down the autoclave to room temperature and store the        suspension.        This suspension contained about 1.0% w/w Mg₂Al(OH)₆Cl.H₂O, with        the product yield being about 60%.

The particle size distributions of the suspensions obtained in this runtogether with an example that was heated at 80° C. for 8 and 16 hr areshown in FIG. 9. The average particle size and the zeta potential areshown in Table 5.

TABLE 5 Sample Preparation Peak size Zeta Potential Conditions (nm) (mV)80° C., 8 h 85 ± 20 38.5 80° C., 16 h 89 ± 19 36.9

The present invention provides a suspension of LDH particles in waterand a method for preparing such a suspension. In some embodiments, thesuspension is stable for extended periods of time and will typically notexhibit any separation or segregation for up to a month from formation.Suitably, such stable suspensions will not exhibit separation orsegregation for a period of up to 6 months from formation. Thesuspension includes LDH particles having a narrow particle sizedistribution, with the largest particle dimension of the particlespredominantly falling within the range of 20-400 nm, more suitably40-300 nm. The suspension may contain up to 10% w/w LDH particles,although the suspensions more preferably contain 1% w/w or less LDHparticles. Unlike Liu et al, which shakes and heats the suspension atatmospheric pressure, the present invention utilises a hydrothermalheating step. The present invention also uses higher heatingtemperatures and, in some embodiments, different conditions in theco-precipitation step, when compared to Liu et al. In particular, Liu etal is specific in directing the reader to age the precipitated particlesand alkaline solution formed in the precipitation step for 1 hour. Incontrast, the present inventors have found that the precipitatedparticles and the solution remaining after precipitation shoulddesirably remain in contact with each other for not longer than 30minutes.

The suspensions in accordance with the present invention may be used inbiomedical applications, for example, to prepare bio-inorganic hybridcomposites as described in European patent application no. 987328, theentire contents of which are herein incorporated by cross reference. Thesuspension may be also used to manufacture polymer/clay nanocompositesfor membrane separation, biomedical materials and other uses. Thesuspensions may also be useful as a component in the manufacture ofpolymers, with the LDH nanoparticles acting as a filler in the polymer.

The current invention can be used to make, for example LDH where theinterlayer anion is chloride, nitrate, sulfate, and carbonate. Where thesuspension is to be used in an application that requires exchange of theinterlayer anion, LDH-carbonate is not preferred, but it is a goodcandidate to make polymer-LDH nanocomposite as well as polymer fillers,retardants etc. In some cases, LDH-CO₃ can be exchanged with CF, NO₃ ⁻and SO₄ ²⁻ in a lightly acidic solution.

If it is desired to make LDH that is free of carbonate as the interlayeranion, it is preferred to conduct the co-precipitation step in an inertatmosphere, such as in a nitrogen atmosphere, because air containscarbon dioxide that may be absorbed by the solutions, leading tocarbonate ions going into the interlayer space, or in a carbon dioxideor carbonate free environment.

Those skilled in the art will appreciate that the present invention maybe susceptible to variations and modifications other than thosespecifically described. It is to be understood that the presentinvention encompasses all such variations and modifications that fallwithin its spirit and scope.

1. A method for preparing a suspension of LDH particles comprising thesteps of: a) preparing LDH precipitates by coprecipitation to form amixture of LDH precipitates and solution; b) separating the LDHprecipitates from the solution; c) washing the LDH precipitates toremove residual ions; d) mixing the LDH precipitates with water; and e)subjecting the mixture of LDH particles and water from step (d) to ahydrothermal treatment step by heating to a temperature of from greaterthan 80° C. to 150° C. for a period of about 1 hour to about 144 hoursto form a well dispersed suspension of LDH particles in water whereinsaid LDH particles in suspension comprise platelets having a maximumparticle dimension of up to 400 nm.
 2. A method as claimed in claim 1wherein step (e) comprises subjecting the mixture of LDH particles andwater from step (d) to a hydrothermal treatment step by heating to atemperature of from greater than 80° C. to 150° C. for a period of about1 hour to about 2 hours.
 3. A method as claimed in claim 1 wherein theplatelets exhibit a particle size distribution of no more than about±27%, based upon the peak value and the peak width at half maximum fromthe particle size distribution of an intensity average using photoncorrelation spectroscopy (PCS) measurement.
 4. A method as claimed inclaim 1 wherein the LDH precipitates and the solution formed in step (a)are left in contact with each other for a period not exceeding 30minutes.
 5. A method as claimed in claim 1 wherein the LDH precipitatesand the solution formed in step (a) are left in contact with each otherfor a period not exceeding 20 minutes.
 6. A method as claimed in claim 1wherein the LDH precipitates and the solution formed in step (a) areleft in contact with each other for a period not exceeding 10 minutes.7. A method as claimed in claim 1 wherein the LDH precipitates and thesolution formed in step (a) are left in contact with each other for aperiod not exceeding 5 minutes.
 8. A method as claimed in claim 1wherein the LDH precipitates and the solution formed in step (a) areleft in contact with each other for a period not exceeding 1 minute. 9.A method as claimed in claim 1 wherein the LDH precipitates and solutionformed in step (a) are stirred.
 10. A method as claimed in claim 1wherein the hydrothermal treating step is carried out while suppressingboiling.
 11. A method as claimed in claim 1 wherein in step (a) a mixedmetal ion solution and an alkaline solution are rapidly mixed together.12. A method as claimed in claim 11 wherein the mixed metal ion solutionand the alkaline solution are added together within a time period ofless than 1 minute.
 13. A method as claimed in claim 1 whereinprecipitation in step (a) takes place at a temperature ranging from roomtemperature up to about 50° C.
 14. A method as claimed in claim 1wherein the LDH precipitates are washed one or more times with deionizedwater following separation from the solution.
 15. A method as claimed inclaim 1 wherein the mixture of water and LDH precipitates is held at theelevated temperature for a period of from 2-48 hours in step (e).
 16. Amethod as claimed in claim 14 wherein the mixture of water and LDHprecipitates is held at the elevated temperature for a period of from1-24 hours.
 17. A method as claimed in claim 1 wherein the largestdimension of the platelets in the suspension predominantly falls withinthe range of 20-400 nm.
 18. A method as claimed in claim 1 wherein thelargest dimension of the platelets in the suspension predominantly fallswithin the range of 40-300 nm.
 19. A method as claimed in claim 1wherein the suspension contains up to 10% w/w LDH platelets.
 20. Amethod as claimed in claim 1 wherein the suspension contains up to 5%w/w LDH platelets.
 21. A method as claimed in claim 1 wherein thesuspension contains up to about 1% w/w LDH platelets.
 22. A method asclaimed in claim 1 wherein the suspension contains less than 1% w/w LDHparticles.
 23. A method as claimed in claim 1 further comprising thestep of removing at least some of the water from the suspension toconcentrate the particles to form a more concentrated suspension.
 24. Amethod as claimed in claim 1 wherein step (d) comprises mixing the LDHprecipitates with water in order to disperse the LDH precipitates in thewater and to break up any loose aggregates of LDH precipitates intoplatelets.
 25. A method for forming LDH particles comprising forming asuspension as claimed in claim 1 and separating the LDH particles fromthe suspension.